The present invention relates to a method and apparatus for the production of elongated carbonaceous articles. The present invention has particular applicability in manufacturing carbonaceous articles, such as carbon fibers and carbon nanotubes in high yields and throughput.
Carbonaceous materials, in general, enjoy wide utility due to their unique physical and chemical properties. Recent attention has turned to the use of elongated carbon-based structures, such as carbon filaments, carbon tubes, and in particular nanosized carbon structures. It has been shown that these new structures impart high strength, low weight, stability, flexibility, good heat conductance, and a large surface area for a variety of applications.
Of growing commercial interest is the use of single-wall carbon nanotubes to store hydrogen gas, especially for hydrogen-powered fuel cells. It is anticipated that hydrogen-powered fuel cells can offer advantages over traditional gasoline powered transportation. By current estimates, a hydrogen-powered automobile would require about 3 kg of hydrogen gas to have the equivalent 400 mile driving range as that of conventional gasoline powered automobiles. Since it is believed that nanotubes can store approximately 7-8% of hydrogen by weight relative to carbon, a hydrogen-powered automobile would require approximately 40 kg of carbon nanotubes to store a sufficient amount of hydrogen to power the typical automobile. The best-known techniques, however, for producing single-wall carbon nanotubes and carbon fibers produce only approximately 4 g of carbon nanotubes per day at a prohibitively high cost.
The formation of carbon filaments through catalytic decomposition of hydrocarbons is known. For example, U.S. Pat. No. 5,165,909 to Tennent et al. disclose the production of carbon fibrils characterized by a substantially constant diameter and a length greater than about 5 times the diameter by continuously contacting metal particles with a gaseous, carbon-containing compound to catalytically grow the fibrils. European Patent 56,004B1 to Yates et al. discloses methods of preparing iron oxides for the production of carbon filaments. U.S. Pat. No. 5,780,101 to Nolan et al. discloses methods of producing highly crystalline nanotubes by the catalytic disproportionation of carbon monoxide in the substantial absence of hydrogen.
U.S. Pat. Nos. 5,872,422 and 5,973,444 both to Xu et al. disclose carbon fiber-based field emission devices, where carbon fiber emitters are grown and retained on a catalytic metal film as part of the device. Xu et al. disclose that the fibers forming part of the device may be grown in the presence of a magnetic or electric field, as the fields assist in growing straighter fibers.
One particular problem associated with conventional carbon-fiber forming techniques is that the catalysts used to facilitate the production of carbon nanotubes or carbon fibers often migrate with the growth of the nanotubes and contaminate the produced products. Therefore, the carbon fibers or carbon nanotubes must be treated, such as with nitric acid, to remove the catalyst and purify the products. This treatment, of course, impedes the production process of carbon-based products and chemically destroys a significant portion of the production of carbon nanotubes (up to 80-90% by some estimates). Also, the catalyst is lost during the process since it is not reusable after acid treatment and, thus, yield of carbon per catalyst particle is particularly low.
Accordingly, a need exists for the efficient manufacture of carbonaceous articles, in particular nanosized carbon-based articles in high yield, throughput and purity.
An advantage of the present invention is an apparatus for producing carbonaceous articles in high yield, purity and efficiency.
Another advantage of the present invention is a method of manufacturing carbonaceous articles with high efficiency and a reduced need for further purification.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will be apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by an apparatus for manufacturing carbonaceous articles. The apparatus comprises a chamber having at least one heating element and at least one port for introducing a precursor to the chamber. The heating element can be any element useful for heating the contents of the chamber and the port can be a gas inlet port, for example. A metal catalyst is disposed in the chamber that is capable of converting the introduced precursor to a carbonaceous article. The catalyst can be in free form or supported in the chamber.
In accordance with the present inventive apparatus, a device is positioned near the metal catalyst that is capable of generating a magnetic field. The magnetic field is useful for affecting or influencing the catalyst as by substantially inhibiting the mobility of the metal catalyst during the formation of the carbonaceous article from the precursor. The apparatus of the present invention advantageously restricts the mobility of the catalyst thereby reducing contamination of the produced products and improves efficiency and yield of products per catalyst by reducing the loss of the catalyst in the products.
Embodiments of the present invention include an apparatus comprising a second chamber disposed within the chamber; a catalyst bed disposed in the chamber; and a stationary magnet at a distance so as to influence the catalyst bed, wherein the catalyst bed comprises the metal catalyst supported on a porous substrate and wherein the metal catalyst comprises a nickel, cobalt or iron-based catalyst or mixtures thereof.
Another aspect of the present invention is a method of a manufacturing a carbonaceous article, e.g. a carbon nanotube. The method comprises contacting a carbon-containing precursor with a catalyst to form the carbonaceous article; applying a magnetic field near the catalyst during the formation of the carbonaceous article; and separating the formed carbonaceous article from the catalyst.
The inventive method advantageously produces carbonaceous articles, such as carbon fibers and tubes, without the need for further purification thereby minimizing product loss due to purification processes. By magnetically preventing catalyst migration, yield is also improved reducing the need to re-seed the catalyst thereby improving throughput.
Embodiments of the present invention comprise forming a nanostructured carbonaceous article by contacting a carbon-containing precursor with a nanosized metal catalyst at elevated temperatures, e.g. from about 100xc2x0 C. to about 1000xc2x0 C., while applying a magnetic field of at least about 100 gauss near the catalyst.
Another aspect of the present invention is a method of a using a catalyst for producing carbonaceous articles, the method comprising contacting a carbon-containing precursor with a catalyst bed to form a first carbonaceous article; applying a magnetic field near the catalyst bed during the formation of the first carbonaceous article; separating the formed first carbonaceous article from the catalyst bed and reusing the catalyst to form a second carbonaceous article. The method of the present invention advantageously reduces the need to re-seed the catalyst bed thereby increasing the efficiency of the process.
Another aspect of the present invention is a carbonaceous article, e.g. a nanosized carbon fiber or tube, having an elongated portion with an aspect ratio of no less than 2 and having opposing proximal and distal ends. Embodiments include where the elongated portion and ends comprise no less than 90 atomic percent (at. %) of carbon and are substantially free of catalyst, e.g. metals and their salts. The carbonaceous articles of the present invention can be formed having less than 10 weight percent (wt %) of metal impurities, e.g. less than about 5 wt % of metal impurities, without the need for acid purification.